1. Technical Field
The present disclosure relates to a resonant microelectromechanical structure (MEMS) with improved electrical characteristics, in particular for real-time-clock (RTC) applications, to which the following treatment will make reference without this implying any loss of generality.
2. Description of the Related Art
As is known, RTC devices are commonly used operating as a clock in electronic apparatuses, whether portable or not, such as for example cell phones, video cameras or photographic cameras, automotive apparatuses, electrical household appliances, data-collection terminals, smart-card readers, etc., in order to count the passage of real time (in terms of years, months, days, hours, minutes, and seconds), even when the corresponding electronic apparatuses are turned off. An RTC device generally comprises for the purpose: an oscillator circuit provided with an appropriate resonant structure designed to generate an operating frequency (or resonance frequency), typically of 32.768 kHz; a processing circuit, coupled to the oscillator circuit, for counting the passage of time on the basis of the same operating frequency; and an appropriate power-supply source for providing the electrical supply to the device.
Even though quartz technology has dominated for decades the field of frequency generation (also for real-time-clock applications), in recent times silicon-based MEMS resonators have been proposed, with increasingly greater success. The advantages linked to the use of MEMS resonators are represented above all by a marked reduction in the dimensions and by the considerable reduction in costs, thanks to the possibility of producing the MEMS resonators with standard processes of manufacture of integrated circuits, and to the possibility of integrating at low cost in one and the same chip both the mechanical structure and the corresponding electronic circuit (in the form of ASIC—Application Specific Integrated Circuit).
MEMS resonators include micromechanical structures obtained by means of micromachining techniques, which, due to external stresses (in the form of appropriate electrical biasing), are induced to vibrate at their natural resonance frequency. These micromechanical structures comprise a mobile mass, anchored to a substrate by means of purposely provided mechanical-constraint elements, which is set in resonance. The mobile mass forms, with a fixed-electrode structure coupled thereto, a capacitor, and the vibration in resonance conditions of the mobile mass causes a variation of the capacitance of this capacitor, which is converted into an output signal at the desired operating frequency.
In particular, the solutions up to now proposed for providing silicon MEMS resonators are represented by structures of a lateral type, i.e., ones in which the mobile mass, suspended above the substrate by means of appropriate constraint elements, vibrates in a direction parallel to the same substrate.
The capacitive coupling between the mobile mass and the fixed-electrode structure, which faces it during oscillation, can be obtained with a configuration of a comb-fingered type or with a parallel-plate configuration.
In the comb-fingered configuration (see for example W. C. Tang, T.-C. H. Nguyen, R. T. Howe, “Laterally driven polysilicon resonant microstructures”), the use of a large number of fingers associated to, and extending from, the mobile mass, and of corresponding fixed electrodes facing them, enables a high capacitive coupling to be obtained between the mobile mass and the fixed electrodes. However, it has been shown that it is impossible to regulate the operating frequency of the device (to achieve the so-called “tunability”), and hence also to correct the shifts in the same frequency due to the spread of the technological process and to the temperature shifts. In particular, it has been demonstrated the intrinsic invariance of the resonance frequency with respect to the biasing voltage applied to the electrodes.
In the parallel-plate configuration (see, for example, US 2008/0186109 A1), it is the same mobile mass that constitutes, with its surfaces facing and parallel to corresponding surfaces of the fixed electrodes, one of the plates of the capacitor, the capacitive variation of which is used for generation of the output signal at the desired operating frequency. In this case, it has been shown the possibility of compensating for the possible shifts in the resonance frequency, via variation of the biasing potential applied between the mobile mass and the fixed electrodes. In fact, the variation of the biasing voltage enables modification of the electrostatic force acting on the resonant structure, modifying the state of stress and hence the effective elastic constant. However, this advantage is counterbalanced by the difficulty of obtaining in this case an efficient capacitive coupling, and hence a low equivalent resistance (the so-called “motional resistance”) of the mechanical structure.
This drawback is due chiefly to technological limits (linked to the photolithographic technologies used for the definition of the mobile mass) in providing gaps that can be controlled and are sufficiently small (less than a micron) between the mobile mass and the fixed electrodes over the entire wafer, and a facing area that is sufficiently large between the same elements. The lateral gap between the mobile mass and the fixed electrodes is in fact constrained by the minimum resolution of the photolithographic-etching process, whereas the facing area is limited by the reduced thickness of the mobile mass in a direction orthogonal to the substrate.
In greater detail, it may be shown that the equivalent, or motional, resistance Rm of the mechanical structure of the resonator is given by the following expression:
                              R          m                =                                            k              eq                        ·                          d              4                                                          ω              0                        ·            Q            ·                          V              p              2                        ·                          ɛ              0              2                        ·                          A              2                                                          (        1        )            where keq is the equivalent elastic constant of the resonant structure, ω0 is the natural resonance pulse (with ω0=√{square root over (keq/meq)}, where meq is the equivalent mass of the mobile mass), Q is the quality factor, Vp is the biasing voltage applied between the mobile mass and the fixed-electrode structure, ε0 is the vacuum permittivity, A is the capacitive coupling surface area, and d is the gap between the mobile mass and the fixed-electrode structure. The motional resistance Rm is hence directly proportional to the fourth power of the gap d, and inversely proportional to the square of the facing surface A.